Efficient transmission of digital return path data in cable television return path

ABSTRACT

An efficient means for transmitting digitized return path signals over a cable television return path is disclosed. In one embodiment of the invention, the cable television return path includes a node that receives an analog return signal from a subtree of the cable television system and generates a digital transport signal representative of the analog return path signal. The digital transport signal, however, is not a digitized form of the analog return signal. Rather, the digital transport signal is encoded such that fewer bits are used to represent the analog return signal without substantially impacting the accuracy and dynamic range of the signal. At the hub, the digital transport signal is decoded and converted to produce an analog signal that is a close approximation of the analog return signal.

[0001] The present application claims priority, under 35 U.S.C. 119(e),to U.S. Provisional Patent Application Serial No. 60/356,955, bearingattorney docket number 9775-0078-888, filed Feb. 12, 2002, which isincorporated herein by reference. The present application also claimspriority, under 35 U.S.C. 120, to U.S. patent application Ser. No.10/102,625, bearing attorney docket number 9775-0078-999, filed Mar. 19,2002, which is also incorporated herein by reference. Both applicationsto which priority is claimed have the same title as above.

BRIEF DESCRIPTION OF THE INVENTION

[0002] The present invention is related generally to upstream datacommunications over networks primarily designed for downstreamtransmission of television and data signals. More specifically, thepresent invention pertains to a device and method for improvingperformance of digital return path links for a cable television (CATV)hybrid fiber co-axial cable (coax) system.

BACKGROUND OF THE INVENTION

[0003] Cable television systems (CATV) were initially deployed so thatremotely located communities were allowed to place a receiver on ahilltop and then use coaxial cable and amplifiers to distribute receivedsignals down to the town which otherwise had poor signal reception.These early systems brought the signal down from the antennas to a “headend” and then distributed the signals out from this point. Since thepurpose was to distribute television channels throughout a community,the systems were designed to be one-way and did not have the capabilityto take information back from subscribers to the head end.

[0004] Over time, it was realized that the basic system infrastructurecould be made to operate two-way with the addition of some newcomponents. Two-way CATV was used for many years to carry back somelocally generated video programming to the head end where it could beup-converted to a carrier frequency compatible with the normaltelevision channels.

[0005] Definitions for CATV systems today call the normal broadcastdirection from the head end to the subscribers the “forward path” andthe direction from the subscribers back to the head end the “returnpath.” A good review of much of today's existing return path technologyis contained in the book entitled Return Systems for Hybrid Fiber CoaxCable TV Networks by Donald Raskin and Dean Stoneback, herebyincorporated by reference as background information.

[0006] One innovation, which has become pervasive throughout the CATVindustry over the past decade, is the introduction of fiber opticstechnology. Optical links have been used to break up the original treeand branch architecture of most CATV systems and to replace that with anarchitecture labeled Hybrid Fiber/Coax (HFC). In this approach, opticalfibers connect the head end of the system to neighborhood nodes, andthen coaxial cable is used to connect the neighborhood nodes to homes,businesses and the like in a small geographical area.

[0007]FIG. 1 is a block diagram of a digital return path 100 of a priorart cable television system that uses conventional analog return pathoptical fiber links. As shown, analog return signals, which includesignals generated by cable modems and set top boxes, are present on thecoaxial cable 102 returning from the customer. The coaxial cable 102 isterminated at a node 110 where the analog return signals are convertedto a digital representation by an A/D converter 112. The digital signalis used to modulate an optical data transmitter 114 and the resultingoptical signal is sent over an optical fiber 106 to a hub 120. At thehub 120, the optical signal is detected by an optical receiver 122, andthe detected digital signal is used to drive a D/A converter 124 whoseoutput is the recovered analog return signals.

[0008] The analog return signals present on the coaxial cable 102 aretypically a collection of independent signals. Some of these independentsignals may have high peak values and some of the signals may be lowlevel signals. To detect the low level analog return signals and toaccommodate the high level analog return signals at the same time, anA/D converter with a large number of bits (e.g., a 10-bit A/D converter)is typically used in the node 110. In the United States, because theanalog return signals are in the frequency range of 5 to 42 MHz, thesampling rate of the A/D converter is typically about 100 MHz. A 10-bitA/D converter operating at a sample rate of 100 MHz will output data ata rate of 1 Gbps. Therefore, optical transmitters and the opticalreceivers in an CATV optical link must be capable of transmitting andreceiving optical signals at 1 Gbps or at a higher rate. Naturally, thecosts of such high-speed optical equipment are high. Limits on thebandwidth of the optical equipment also restrict the number of analogreturn signals that can be bundled together for transmission on the sameoptical fiber.

[0009] Accordingly, there exists a need for a system and method fortransmitting digital data on the CATV return path at a rate that islower than a full rate without significant loss of performance.

SUMMARY OF THE INVENTION

[0010] An embodiment of the present invention is a cable televisionreturn path at which analog return signals are converted to digitalformat and encoded, and then transmitted across an optical link to ahub. At the hub, the encoded digital signal is decoded and converted toproduce an analog signal that is a close approximation of the analogreturn signal. The encoding scheme of the present embodiment isefficient in reducing the number of bits that are transported across theoptical link. Yet, the accuracy of the analog return signal is notsubstantially compromised. Furthermore, the encoding scheme of thepresent embodiment is simple, suitable for high-speed operations andcost-effective.

[0011] In one embodiment, the cable television return path includes anode that receives an analog return signal from a subtree of the cabletelevision system and generates a digital transport signalrepresentative of the analog return path signal. The digital transportsignal, however, is not a digitized form of the analog return signal.Rather, the digital transport signal is encoded such that fewer bits areused to represent the analog return signal without substantiallyimpacting the accuracy and dynamic range of the signal. At the hub, thedigital transport signal is decoded and converted to produce an analogsignal that is a close approximation of the analog return signal.Because the digital transport signal has fewer bits per sample, areduced number of bits will be transmitted across the optical link, thusallowing the optical link to operate at a lower transmission rate. Atthe same transmission rate, a larger number of return signals can becommunicated across the optical link. This means that a larger number ofanalog return links can be bundled together and transported across theoptical link.

[0012] In one particular embodiment, the node of the cable televisionreturn path includes an N-bit A/D converter, an optical transmitter, andan encoder coupled between the A/D converter and the opticaltransmitter. The encoder is configured to receive N-bit digital samplesfrom the A/D converter and to generate digital samples with fewer than Nbits per sample. Each sample includes a sign bit. In this embodiment,the encoder determines a size of each of the N-bit digital samples, andgenerates selection bits for each sample indicative of the sample'sdetermined size. In one example, there are four different sizes: smallpositive and negative, medium, and large, covering eight possible rangesof an N-bit sample (the medium size covers both positive and negativevalues, and the large size covers two ranges for both positive andnegative values). Based on the size, the encoder outputs a transportsample that includes the selection bits and a subset of the N-bits ofthe digital sample, such as the least-significant bits for a smallsample. For other samples sizes, such as a large sample, the transportsample may include a subset of the N-bits that does not include, forexample, the two least significant bits The output of the encoder isthen passed to the optical transmitter to be converted to an opticalsignal for transmission to the hub.

[0013] In this embodiment, the hub of the cable television return pathincludes an optical receiver, a D/A converter, and a decoder coupledbetween the optical receiver and the D/A converter. At the hub,transport samples are recovered from the optical signals and provided tothe decoder, which uses the selection bits of each transport sample togenerate a representation of the one or more selection bits. Therepresentation of the selection bits is combined with the non-selectionbits of the transport sample (or a subset of the non-selection bits),and padded with zeros, if necessary, to generate an N-bit sample. TheN-bit sample output from the decoder is then provided to the D/Aconverter to be converted into an analog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Aspects of the present invention will be more readily apparentfrom the following description and appended claims when taken inconjunction with the accompanying drawings, in which:

[0015]FIG. 1 is a block diagram depicting a cable television (CATV)digital return path of the prior art.

[0016]FIG. 2 is a block diagram depicting a CATV return path accordingto one embodiment of the present invention.

[0017]FIG. 3 is a block diagram depicting an encoder that can be used inthe CATV return path of FIG. 2, in accordance with one embodiment of thepresent invention.

[0018]FIG. 4A depicts a relationship between the A/D bits, Ax, of alarge sample to the transport bits, Tx, generated by the encoder of FIG.3.

[0019]FIG. 4B depicts a relationship between the A/D bits of a smallsample to the transport bits generated by the encoder of FIG. 3.

[0020]FIG. 5 is a block diagram depicting a decoder that can be used inthe CATV return path of FIG. 2, in accordance with one embodiment of theinvention.

[0021]FIG. 6A depicts a relationship between the transport bits, Tx, andthe D/A bits, Dx, of a large sample generated by the decoder of FIG. 5.

[0022]FIG. 6B depicts a relationship between the transport bits, Tx, andthe D/A bits, Dx, of a small sample generated by the decoder of FIG. 5.

[0023]FIG. 7 is a block diagram depicting another encoder that can beused in the CATV return path of FIG. 2, in accordance with anotherembodiment of the present invention.

[0024]FIG. 8 is a block diagram depicting another decoder that can beused in the CATV return path of FIG. 2, in accordance with an embodimentof the present invention.

[0025]FIG. 9A depicts a relationship between the A/D bits, Ax, thetransport bits, Tx, and the D/A bits, Dx, of a small positive samplegenerated by the decoder of FIG. 8.

[0026]FIG. 9B depicts a relationship between the A/D bits, Ax, thetransport bits, Tx, and the D/A bits, Dx, of a small negative samplegenerated by the decoder of FIG. 8.

[0027]FIG. 9C depicts a relationship between the A/D bits, Ax, thetransport bits, Tx, and the D/A bits, Dx, of a medium sample generatedby the decoder of FIG. 8.

[0028]FIG. 9D depicts a relationship between the A/D bits, Ax, thetransport bits, Tx, and the D/A bits, Dx, of a large sample generated bythe decoder of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]FIG. 2 is a block diagram depicting a CATV return path 200according to one embodiment of the present invention. At the CATV returnpath transmitter 210, a signal encoder 213 is coupled to receive andencode the data signal output of the A/D converter 112. The encoded datasignal (output by the encoder 213) is provided to the opticaltransmitter 114 for transmission to a hub 220. At the hub 220, thesignal receiver 122 recovers and generates a local replica of theencoded data signal, and a signal decoder 223 is coupled to receive anddecode the encoded data signal. The output of the decoder 223, which isa decoded data signal, is provided to a D/A converter 124 for conversioninto analog signals. In this embodiment, the signal encoder 213 andsignal decoder 223 allow digital data to be transmitted across theoptical link at a lower rate than N*F bits per second (where N is thenumber of bits and F is the sampling frequency of the A/D converter 112)without significant loss of performance.

[0030] One implementation of the signal encoder 213 is shown in FIG. 3.In the present example, the input signal from the A/D converter areN-bit samples Ax with bit A_(N−1), being the most significant and bit AOthe least significant. With 2's complement representation of the value,bit A_(N−1) is a sign bit and bits A_(N−2) to A₀ are the amplitude withextended sign. The output of the encoder 213 is N−1 bit samples to betransported. These N−1 bit samples are referred to herein as transportsamples.

[0031] With reference still to FIG. 3, bits A_(N−2) and A_(N−3) areexamined to determine if the amplitude of the N-bit sample is largerthan the maximum value of the A/D converter divided by four. If theamplitude is larger than the maximum value of the A/D converter dividedby four, the selection bit output of the signal encoder 213 is set to a‘1’. If the amplitude is equal to or less than the maximum value of theA/D converter divided by four, then the selection bit is set to a ‘0’.If the N-bit sample is a positive number and either A_(N−2) or A_(N−3)is equal to ‘1’, the selection bit is set to ‘1’. If the N-bit sample isa positive number and both A_(N−2) and A_(N−3) are equal to ‘0’, theselection bit is set to ‘0’. If the N-bit sample is a negative numberand either A_(N−2) or A_(N−3) is equal to ‘0’, the selection bit is setto ‘1’. If the N-bit sample is a negative number and both A_(N−2) andA_(N−3) are equal to ‘1’, the selection bit is set to ‘0’.

[0032] If the selection bit is ‘1’, a “large” sample is present and themost significant N−2 bits (i.e., A_(N−1) to A₂) are transmitted with theselection bit concatenated for a total of N−1 bits. As an example,consider a “large” 10-bit A/D sample, Ax, and a corresponding 9-bittransport sample, Tx, shown in FIG. 4A. This “large” 10-bit A/D samplehas an amplitude larger than the maximum value of the A/D converterdivided by four. As shown, the selection bit ‘1’ and the mostsignificant N−2 bits (bit A₉ to A₂) of Ax are mapped to bits T₈ to T₀ ofTx.

[0033]FIG. 4B illustrates a relationship between a “small” 10-bit A/Dsample, Ax, and a corresponding transport sample, Tx. As shown, theleast significant N−3 bits (e.g., A₀ to A_(N−4)), together with the signbit and the selection bit ‘0’, are mapped to bits T₀ to T₈ of Tx. Thatis, for a “small” sample whose amplitude is smaller than the maximumvalue of the A/D converter divided by four, the transmitted bits are: A₀to AN₄, A_(N−1) and the selection bit ‘0’ for a total of N−1 bits.

[0034]FIG. 5 is a block diagram depicting the signal decoder 223according to one embodiment of the present invention. The input signalis the transport sample, Tx, with N−1 bits. The input signal, Tx, is alocal replica of the encoded signal. The output is an N bit sample, Dx,for conversion into analog signals by the D/A converter 124.

[0035] In operation, the signal decoder 223 examines the transportsample, Tx. If the selection bit is a ‘1’, a “large” sample has beenreceived. The signal decoder 213 then removes the selection bit T₈. TheD/A sample, Dx, is constructed by mapping the bits T₀ through T_(N−3) tothe most significant bits of Dx and padding the least two significantbits of Dx with ‘1’ and ‘0’. As an example, a “large” D/A sample, Dx,constructed from a transport sample Tx is shown in FIG. 6A. In someother embodiments the “large” D/A sample is padded with bits other than‘10’.

[0036] If the selection bit is a “0”, a small sample has been received.The signal decoder 223 removes the selection bit T_(N−2) (T_(N−2) is T₈in our example using 10-bit samples). The D/A sample Dx is constructedby mapping the bits T₀ through T_(N−3) to the least significant D₀through D_(N−3) bits and extending the sign bit T_(N−3) (T₇ in ourexample) to fill the D_(N−1) through D_(N−2) bits of the sample Dx. Asmall D/A sample, Dx, constructed from a transport sample Tx is shown inFIG. 6B.

[0037] A preferred embodiment of the present invention, a CATV returnpath 200 that transports 10-bit A/D samples of a CATV return path signalin 9-bit transport samples, has been described above. In anotherembodiment, the CATV return path transports return path signals usingA/D samples and transport samples that have a different member of bits.

[0038] Furthermore, in an embodiment described above, the selection bitgives an offset of two bits between the large and small samples. Inother embodiments of the invention, the selection bit may provide anoffset of any number of bits between a large and a small sample. Forinstance, in one embodiment, a selection bit of ‘1’ indicates that N−kmost significant bits of the A/D samples are transported, and aselection bit of ‘0’ indicates that the N−(k+1) least significant bitsof the A/D samples and their sign bits are mapped to the transportsamples. In that embodiment, each transport sample will include N−k+1bits, for a saving of k−1 bits per sample.

[0039] In yet another embodiment, multiple selection bits are used. Forthis embodiment, the range of possible values for a N-bit digitalsample, i.e., the maximum positive value to the maximum negative value,is divided into eight equal size sub-ranges, including four positive andfour negative. The smallest positive and negative ranges of values aredesignated as “small” and, more specifically, as either “small positive”or “small negative.” The next largest range of values, both positive andnegative, is designated as “medium.” The next two largest ranges ofvalues, positive and negative, are designated as “large.” Based on thesize of the sample (small positive, small negative, medium, or large), a2-bit code is generated. The 2-bit code is concatenated with a number ofbits from the original sample to create a transport sample determined bythe value of the 2-bit code, as will be described next.

[0040] Referring to FIG. 7, this embodiment of the signal encoder 213 isshown. In this embodiment, instead of a 2's complement representation ofthe value, the samples are in offset-binary format. Like a 2'scomplement representation, offset-binary format is another method forrepresenting signed numbers, and also uses bit A_(N−1) as a sign bit andbits A_(N−2) to A₀ as the amplitude. An offset-binary number is derivedby determining the largest-possible value for an N-bit sample, dividingthat value in half, and assigning the quotient to be the zero value. Todetermine a positive offset-binary number, the absolute value of thenumber is added to the zero value, and to determine a negativeoffset-binary number, the absolute value is subtracted from the zerovalue. The net effect is to add an offset equal to half the value of thetotal range.

[0041] For example, the following table shows how to determine anoffset-binary representation of two numbers, +22 and −7, using aneight-bit representation:

[0042] a) largest value for 8-bit integer=2⁸=256

[0043] b) offset-binary zero value=256÷2=128_((decimal))=1000 0000(binary)

[0044] c) 1000 0000_((offset binary 0))+0001 0110_((binary 22))=10010110_((offset binary +22))

[0045] d) 1000 0000_((offset binary 0))−0000 0111_((binary 7))=01111001_((offset binary −7))

[0046] Thus, for a 10-bit sample, the greatest integer than can berepresented in offset-binary is +511 (11 11111111_((offset binary +51))), and the greatest negative number is −511(01 1111 1111_((offset binary −511))), for a range of 1023. Because thisis not divisible into 8 equal parts, a set of seven ranges of 128 each,and one range of 127, are used instead. The size types and ranges are asfollows: Size Type Integer Values Binary-Offset Values Small Positive 0- 10 0000 0000 127 10 0111 1111 Medium Positive 128- 10 1000 0000 25510 1111 1111 Large Positive 256- 11 0000 0000 511 11 1111 1111 SmallNegative  (−1)- 01 1111 1111 (−128) 01 1000 0000 Medium Negative (−129)-01 0111 1111 (−256) 01 0000 0000 Large Negative (−257)- 00 1111 1111(−511) 00 0000 0001

[0047] From these sizes and their corresponding binary-offset values, acorrelation can be made that the first three bits dictate what size asample will be. In other words, the encoder can examine bits A_(N−1)through A_(N−3) of the sample to determine which of the eight sizes anN-bit sample falls within. Bits A_(N−1) through A_(N−3) are equal to“100” for a small positive sample, “101” for a medium positive sample,and “110” or “111” for a large positive sample. Similarly, bits A_(N−1)through A_(N−3) are equal to “011” for a small negative sample, “010”for a medium negative sample, and “000” or “001” for a large negativesample. These bits are also referred to herein as the prefix bits of thedigital sample.

[0048] Because two bits are used for the selection code in thisembodiment, only four types of sizes can be coded. Thus, positive andnegative sign values are only coded for small samples (i.e., smallpositive and small negative), whereas medium and large sample values arenot coded with a sign value.

[0049] In this embodiment, if the N-bit sample is small negative orsmall positive, bits A_(N−4) through A₀ are concatenated with the 2-bitselection code and transmitted as a N−1 bit transport sample T_(X). Ifthe N-bit sample is medium, bits A_(N−3) through A₁ are concatenatedwith the 2-bit selection code and transmitted, for a total of N−1 bits.Bit A₀ is not transmitted in this case, and is thus lost. Finally, ifthe N-bit sample is large, bits A_(N−2) through A₂ are concatenated withthe 2-bit selection code and transmitted, again as N−1 bit transportsample T_(X). In this case, bits A₁ and A₀ are not transmitted and aretherefore lost.

[0050] Thus, referring back to FIG. 7, this logic flow is shown for theencoder 213. The encoder receives an N-bit A/D sample, and divides itinto four possible subsets: the bits used for selection coding (bitsA_(N−1) through A_(N−3)); the bits for a small sample (bits A_(N−4)through A₀); the bits for a medium sample (bits A_(N−3) through A₁); andthe bits for a large sample (bits A_(N−2) through A₂). The subsetscorresponding to the various sample sizes are provided to a multiplexor.The bits for selection coding are provided to logic that generates the2-bit selection code, C₁, C₀, in a manner that will be described below.The 2-bit selection code is used as the select signal for themultiplexor, to select from among the various sample subsets, and isthen concatenated with the selected subset. In one embodiment, the 2-bitselection code is concatenated as the most-significant bits of theresultant transport sample, but in other embodiments may be concatenatedas the least-significant bits or may be inserted elsewhere in the samplesubset. The result is the N−1 bit transport sample, T_(X), having bitsT_(N−2) through T₀.

[0051] As shown in FIG. 8, once the N−1 bit transport sample T_(X) hasbeen transmitted and received, the receiving signal decoder 223 receivesa local replica of the transport sample. As described, the 2-bitselection code of the transport sample (bits T_(N−2) and T_(N−3), in oneembodiment) designates whether the sample is small positive, smallnegative, medium, or large. The first three bits of the recreateddigital sample (i.e., D_(N−1), D_(N−2), D_(N−3)) can be determined fromthe 2-bit selection code, plus zero, one or two of the most significantremaining bits (depending on whether the digital sample is small, mediumor large), in accordance with the logic detailed above (e.g., a smallnegative sample contains “011” as the first three bits). Thus, the logicfor generating D_(N−1),D_(N−2), D_(N−3) receives selection code bitsT_(N−2) and T_(N−3) as well as bits T_(n−4) and T_(n−5). The threerecreated bits D_(N−1), D_(N−2), D_(N−3) are also referred to herein asa representation of the selection bits. In other embodiments, therepresentation of the selection bits may be the selection bitsthemselves, with no encoding, or may be more or less bits than thenumber of selection bits, encoded by another encoding scheme.

[0052] The first-three bits of the D/A sample are then concatenated withbits T_(N−4) . . . T₀ of the transport sample (also referred to as thenon-selection bits), or a subset of T_(N−4) . . . T₀, depending on the2-bit selection code, as explained in further detail below. (In anotherembodiment, the representation may be selectively truncated instead).Then, depending on the selection code, the result is padded with therequisite number of padding bits (e.g., “0” bits) as theleast-significant bits. For some selection codes, however, no padding isnecessary. Finally, the result is output as N-bit D/A sample D_(N−1) . .. D₀ for conversion into analog signals by the D/A converter 124.

[0053] The D/A sample D_(X) will be an exact copy of the original A/Dsample A_(X) for small positive and small negative samples. For mediumand large samples, the sample D_(X) will be a close approximation of theoriginal sample A_(X). In the case of a medium sample, theleast-significant bit of D_(X) will be lost, and replaced with a “0”bit. For a large sample, the two least-significant bits of D_(X) will belost and replaced with “0” bits. The added “0” bits are also referred toas padding bits. In an alternate embodiment, the padding bit for mediumsamples is a “1” bit, and the padding bits for large samples are “01”.In yet other alternate embodiments, the padding bits for large samplesare “10” or “11”.

[0054] FIGS. 9A-D show the relationship between the original A/D sample,the transport sample, and the D/A sample. In this example, the A/D andD/A samples are 10 bits each, while the Transport sample is 9 bits. Allfour samples sizes are shown.

[0055]FIG. 9A shows a small positive sample as it passes through theencoder and decoder. If the A/D sample is in offset-binary format, bitsA₉ through A₇ will be “100,” as shown in the table above. This resultsin a 2-bit selection code of “00”, as shown by the first two bits of thetransport sample. Sample bits A₆ through A₀ are concatenated with theselection code. After the transport sample passes through the decoder,the original sample is re-created exactly, such that no bits are lost.

[0056]FIG. 9B shows a small negative sample as it passes through theencoder and decoder. If the A/D sample is in offset-binary format, bitsA₉ through A₇ will be “011.” This results in a 2-bit selection code of“01,” as shown by the transport sample. Again, sample bits A₆ through A₀are concatenated with the selection code. Also, again, after thetransport sample passes through the decoder, the original sample isre-created exactly, such that no bits are lost.

[0057]FIG. 9C shows a medium sample as it passes through the encoder anddecoder. If the A/D sample is in offset-binary format, bits A₉ throughA₇ will be “010” if it is a negative sample, or “101” if it is apositive number. This results in a 2-bit selection code of “10”, asshown in the transport sample. In order to properly distinguish between“medium negative” and “medium positive” samples, bits A₇ through A₁ aresent in the transport sample (even though A₇ is also used in part todetermine the selection code), along with the 2-bit concatenatedselection code. This means that bit A₀ is lost, and will subsequently bereplaced with a “0” bit in the decoder.

[0058]FIG. 9D shows a large sample as it passes through the encoder anddecoder. If the A/D sample is in offset-binary format, bits A₉ throughA₇ will be “000” or “001” if it is a negative sample, or “110” or “111”if it is a positive sample. This results in a 2-bit selection code of“11”, as shown in the transport sample. In order to properly distinguishbetween the four possible large samples, bits A₈ through A₂ are sent inthe transport sample along with the concatenated 2-bit selection code.This means that bits A₁ and A₀ are lost, and will subsequently bereplaced with “0” bits in the decoder.

[0059] In an alternative embodiment that utilizes multiple selectionbits, each combination of selection bits is used to select differentoffsets. For instance, in one embodiment, selection bits of ‘11’indicate that the N−3 most significant bits (e.g., A₈ . . . A₃, whenN=10) of the A/D samples are transported in the transport samples,selection bits of ‘10’ indicate that the N−3 most significant bits otherthan the MSB (e.g., A₈, A₇ . . . A₂) are transported in the transportsamples, and so on.

[0060] In yet another embodiment of the invention, a block of samples(e.g., three consecutive samples) are encoded by the same set ofselection bits. The offset for the largest sample in the block isdetermined first. All samples in the block are then encoded using oneset of selection bits. For instance, consider the example where a blockconsists of three consecutive 10-bit samples, and where a 2-bit offsetbetween “large” samples and “small” samples” is used. In this example,A_(N−2) and A_(N−3) of the largest sample in the block are examined todetermine whether the amplitude of the largest sample in the block islarger than the maximum value of the A/D converter divided by four. Ifso, the N−3 most significant bits of all three samples, including eachsample's sign bit, and one selection bit, are mapped to the transportbits of the transport samples. If not, the N−3 least significant bits ofall three samples, including each sample's sign bit, are mapped to thetransport bits of the transport samples. In this way, even fewer bitsare required to be transported across the optical link, and the opticalreceivers/transmitters can operate at a lower clock rate.

[0061] In the examples described above, 1- or 2-bit selection codes areutilized in the transport sample. In other embodiments, however, theselection code can be an X-bit code, thus separating the range of A/Dvalues in different ways than has been described above. In general, thenumber of distinct possible sizes for an N-bit sample value using anX-bit code is 2^(X). Depending upon the way in which the ranges of A/Dvalues are segmented, and the number of code bits used, different levelsof compression may be achieved along with different degrees of error inthe regenerated signal.

[0062] Also in the example described above, 10-bit A/D and D/A samplesare used. In other embodiments, any size A/D and D/A samples may beused, and the A/D and D/A samples can be different sizes. The techniquedescribed above may be applied in a similar way to smaller or larger A/Dsamples, but will still result in a transport sample that is one bitshorter than the A/D sample. In other embodiments, the transport samplecan be even smaller than N−1 bits, for example N−2 bits, at a cost ofgreater loss of information from the A/D sample.

[0063] While the present invention has been described with reference toa few specific embodiments, the description is illustrative of theinvention and is not to be construed as limiting the invention. Variousmodifications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention. For instance,embodiments of the present invention described above were implemented byhardware logic (e.g., Field Programmable Gate Array(s)). However, aperson of ordinary skill in the art would realize that portions of thepresent invention can be implemented as a program executable by adigital signal processor.

What is claimed is:
 1. A signal encoder for use in a cable televisionreturn path, comprising: an input configured to receive an N-bit digitalinput sample from an analog-to-digital converter; logic configured todetermine a size of the N-bit digital sample, selected from a pluralityof distinct possible sizes of an N-bit value, the logic furtherconfigured to generate one or more selection output bits indicative ofthe determined size of the N-bit digital sample; logic configured toproduce a digital output sample representative of the N-bit digitalinput sample, the digital output sample including a subset of the N-bitsof the digital input sample selected from among a plurality of possiblesubsets for the N-bits, the subset of the N bits being selected inaccordance with the one or more selection output bits, the digitaloutput sample further including the one or more selection output bits,but fewer total bits than N; and an output, coupled to the logic,configured to provide the digital output sample to an opticaltransmitter.
 2. The signal encoder of claim 1, wherein the one or moreselection output bits comprise two selection output bits.
 3. The signalencoder of claim 1, wherein the one or more selection output bitscorrespond to a number of zeros that will be added to the selectedsubset of the N-bits by a decoder to regenerate an approximation of theN-bit digital sample.
 4. The signal encoder of claim 1, wherein theN-bit digital sample is in offset-binary format.
 5. The signal encoderof claim 1, wherein the logic configured to determine a size of theN-bit digital sample determines the size based at least in part on oneor more prefix bits of the digital sample.
 6. A signal decoder for usein a cable television return path, comprising: an input configured toreceive an M-bit digital sample from an optical receiver, the M-bitdigital sample having M bits that include one or more selection bits anda plurality of non-selection bits; logic to generate a representation ofthe one or more selection bits; logic to combine the representation ofthe one or more selection bits, and one or more non-selection bits ofthe plurality of non-selection bits, to create a digital output sample;logic to optionally insert one or more padding bits into the digitaloutput sample, based at least in part on the one or more selection bits;wherein the digital output sample is representative of the M-bit digitalsample, but has more bits than the M-bit digital sample; and adigital-to-analog converter coupled to the logic, the digital-to-analogconverter configured to provide an analog signal corresponding to thedigital output sample.
 7. The decoder of claim 6, wherein the one ormore selection bits comprise a binary-coded value, and further whereinthe representation of the one or more selection bits is an unencodedversion of the binary-coded value.
 8. The decoder of claim 7, whereinthe representation corresponds to a size of the M-bit digital sample. 9.The decoder of claim 6, wherein the one or more non-selection bits thatare combined with the representation include all of the plurality ofnon-selection bits, as determined by the one or more selection bits. 10.The decoder of claim 6, wherein the one or more non-selection bits thatare combined with the representation are a subset of the plurality ofnon-selection bits, as determined by the one or more selection bits. 11.The decoder of claim 6, wherein the logic to optionally insert one ormore padding bits is configured to insert the one or more padding bitsas the least significant bits of the digital output sample.
 12. Thedecoder of claim 6, wherein the M-bit digital sample is in offset-binaryformat.
 13. A cable television return path of a cable television system,comprising: an analog-to-digital converter configured to receive ananalog return signal from a subtree of the cable television system andconfigured to generate a first digital signal representative of theanalog return signal, the first digital signal having a stream of firstdigital samples each having N bits; an encoder configured to receive thefirst digital signal and to generate a second digital signal thatincludes a stream of second digital samples each having a fewer numberof bits than N, each of the second digital samples including one or moreselection bits and a subset of the N-bits of a corresponding one of thefirst digital samples, determined by the one or more selection bits; anoptical transmitter configured to convert the second digital signal intoan optical signal; an optical receiver configured to receive the opticalsignal and to convert the optical signal into a replica of the seconddigital signal; a decoder configured to recover a third digital signalfrom the replica of the second digital signal, the third digital signalincluding a stream of third digital samples, wherein each of the thirddigital samples includes N bits; and a digital-to-analog convertedconfigured to convert the third digital signal to an analog signal thatis substantially equivalent to the analog return signal.
 14. The cabletelevision return path of claim 13, wherein the encoder comprises: aninput configured to receive a digital sample of the first digital signalfrom the analog-to-digital converter; logic configured to determine asize of the N-bit digital sample, selected from a plurality of distinctpossible sizes of an N-bit value, the logic further configured togenerate one or more selection output bits indicative of the determinedsize of the N-bit digital sample; and logic configured to produce adigital output sample representative of the N-bit digital input sample,the digital output sample including a subset of the N-bits of thedigital input sample selected from among a plurality of possible subsetsfor the N-bits, the subset of the N bits being selected in accordancewith the one or more selection output bits, the digital output samplefurther including the one or more selection output bits, but havingfewer total bits than N.
 15. The cable television return path of claim14, wherein the one or more selection output bits generated by theencoder comprise two selection output bits.
 16. The cable televisionreturn path of claim 14, wherein the one or more selection output bitsgenerated by the encoder correspond to a number of padding bits thatwill be added to a corresponding third digital sample recovered by thedecoder.
 17. The cable television return path of claim 13, wherein thedecoder comprises: an input configured to receive a digital sample froman optical receiver, the digital sample having fewer bits than N bitsbut includes one or more selection bits and a plurality of non-selectionbits; logic to generate a representation of the one or more selectionbits; logic to combine the representation of the one or more selectionbits, and one or more non-selection bits of the plurality ofnon-selection bits, to create a digital output sample; and logic tooptionally insert one or more padding bits into the digital outputsample, based at least in part on the one or more selection bits;wherein the digital output sample is representative of the N-bit digitalsample, but has more bits than the N-bit digital sample.
 18. The cabletelevision return path of claim 17, wherein the one or more selectionbits of the digital sample comprise a binary-coded value, and furtherwherein the representation of the one or more selection bits is anunencoded version of the binary-coded value.
 19. The cable televisionreturn path of claim 17, wherein the representation generated by thedecoder corresponds to a size of the M-bit digital sample.
 20. The cabletelevision return path of claim 17, wherein the one or morenon-selection bits that are combined with the representation generatedby the decoder include all of the plurality of non-selection bits, asdetermined by the one or more selection bits.
 21. The cable televisionreturn path of claim 17, wherein the one or more non-selection bits thatare combined with the representation generated by the decoder are asubset of the plurality of non-selection bits, as determined by the oneor more selection bits.
 22. The cable television return path of claim17, wherein the logic to optionally insert one or more padding bits inthe decoder is configured to insert the one or more padding bits as theleast significant bits of the digital output sample.